It will fit a series 5 lens hood from a Retina IIa but is a little tighter than the IIa.
The back threading etc is not similar to the retina IIa front element "stepping" on the IIa

I've wanted a 011 Retina Ektar camera but never bid beyond about $75. Would be nice to compare front elements to one maybe

I don't have a Signet 35 or later series to compare against either. Good camera but not as interesting.